1. Field of the Invention
The present invention relates to a laminate composite comprising a polypropylene sheet (hereinafter referred to as a "core material") made of a modified polypropylene prepared by modifying a crystalline polypropylene with an unsaturated carboxylic acid or a derivative of an unsaturated carboxylic acid and filled with waste paper, an organic fiber or wood pulp at a high concentration or a crystalline polypropylene containing such a modified polypropylene and filled with waste paper, an organic fiber or wood pulp at a high concentration, and metal plates.
Laminate composites made of metal plates and a plastic sheet as the core material in combination have recently been developed from the viewpoint of weight reduction of a metal plate, and acoustically sound absorbing property, vibration damping and heat insulating properties, and widely used in the fields of public works, construction, automobiles and household appliances.
2. Prior Art
A laminate composite using a polypropylene resin as the core material is also partially put into practical use. However, since the resin melts and flows out of a punching portion of a part formed from the composite to cover the punching portion when the part is baked in coating of the part (usually heat-treated at about 180.degree. to 200.degree. C. for 30 min.), troubles are encountered in post-forming. Further, since the resin is shrunken to a large extent in the course of cooling and solidification, particularly the vicinity of shearing work portion is disadvantageously deformed. Thus, the fact is that the above-mentioned laminate composite cannot be used in parts to be subjected to a step of baking finish. With a view to solving the above-mentioned problems concerning the metal-polypropylene laminate composite, the inventors of the present invention previously proposed a laminate composite comprising a sheet made of a blend of a crystalline polypropylene with a specific amount of waste paper or an organic fiber, and metal plates heat- and contact-bonded through modified polypropylene layers to both surfaces of the sheet. Since the heat resistance of the modified polypropylene layers is still insufficient, however, the modified polypropylene layers are softened in baking finish to lower the adhesion thereof to the surfaces of the metal plates, leading to a phenomenon that the vicinity of a cut portion of the composite is partially peeled. Therefore, edge turning or the like of the cut portion is needed in the use of the composite. Thus, the composite is still unsatisfactory in developing the fields of application thereof.
The inventor of the present invention have further made intensive investigations with a view to preventing peeling between the modified polypropylene and the metal surface in high temperature treatment. As a result, they have found that the use as the core material of a polypropylene sheet made of a blend of a modified polypropylene prepared by modifying a crystalline polypropylene with an unsaturated carboxylic acid or its derivative or a crystalline polypropylene containing such a modified polypropylene with a specific amount of waste paper, an organic fiber or wood pulp can provide a metal-polypropylene laminate composite which has an adhesion enough to resist post-forming such as bending, punching, cutting, deep draw forming, etc., and which is free from flowing and dropping out of the molten resin from the cut or punching portion, heat distortion thereof in the vicinity of the cut or punching portion, and peeling in the interface between the resin and the metal even in the course of baking finish. The present invention has been completed based on such a finding.
As is apparent from the above description, an object of the present invention is to provide a highly heat-resistant metal-polypropylene laminate composite which has a high adhesion between the metal and the resin as the core material, and which is free from flowing and dropping out of the molten resin from the cut portion, heat distortion in the vicinity of the cut portion, and peeling between the metal and the core material.